The invention relates generally to extracting compressor air to provide cooling air for a turbine in a gas turbine and, more particularly to establishing control set points for the extraction of compressor air.
In industrial gas turbine engines, air is extracted from one or more stages of the compressor and applied to cool the turbine. The extraction of compressor air is commonly referred to as bleeding air from the compressor. The extracted compressor air is applied as cooling air that passes through internal cooling passages to the turbine blades and buckets.
The air extracted from the compressor for turbine cooling reduces the amount of air flowing through the compressor and to the combustion section of the gas turbine. This reduction in compressed air to the combustor may, at times, have under desirable effects on the performance of the combustor and the total performance of the gas turbine. Control systems are employed to regulate the extraction of compressor air to minimize these undesirable effects on the performance of the gas turbine and ensure that adequate cooling air reaches the turbine blades and buckets.
An approach to regulating the extraction of compressor air is to extract air from two or more stages of the compressor. Air extracted from the lower pressure stage of a compressor tends to have a lesser effect on the performance of the gas turbine than does air extracted from a higher pressure stage. By adjusting the relative proportions of air extracted from the two compressor stages, the control system can reduce or increase the effect of extraction of compressor air on the performance of the gas turbine and provide sufficient cooling air for the turbine.
Air ejectors are conventionally used to combine air at different pressures, such as air extracted from different stages of a compressor. An ejector has been used to combine air from different stages of a compressor to provide turbine cooling air. For example, compressor air has been extracted from a thirteenth stage of the compressor to cool a second stage nozzle of the turbine. Compressor air has also been extracted from a ninth stage of the compressor, where the air extracted from the ninth stage is at a lower pressure and temperature than is the air extracted from the thirteenth compressor stage. The extracted air from the thirteenth stage of the compressor, for example, may be at a pressure and temperature too great for the desired turbine cooling air. By employing an ejector, the low pressure and temperature air extracted from the ninth stage of a compressor is mixed with the high pressure and temperature air extracted from the thirteenth stage to provide an airflow at an intermediate pressure and temperature substantially matching the pressure and temperature required to cool the turbine stage.
An ejector generally does not have moving parts and, thus, does not provide for adjustments with respect to the mixing of air flows. During the design of the gas turbine, the ejector may be sized to provide a turbine cooling air at a desired pressure and temperature. However, the sizing of the ejector may assume that the gas turbine is operating at standard ambient conditions. Daily ambient temperature and pressure variations will impact the operational characteristics of the ejector. The temperature and pressure of air discharged from the ejector will vary as ambient conditions vary. On hot days, the ejector may deliver more cooling air than is required by the turbine. The performance of the compressor unnecessarily suffers because more air is extracted from the compressor than is needed to cool the turbine and, thus, the work required to compress the excess extracted air is wasted. On cold days, the ejector may not deliver enough air to cool the turbine. To account for such cold days, a bypass line has been used to allow some of the extracted air from the thirteenth compressor stage to bypass the ejector and flow directly to the turbine.
A regulatory valve has been provided to adjust the flow of bypass air depending on ambient conditions. A control system determines when to add air extracted from the ninth compressor stage to air extracted from the thirteenth compressor stage in the ejector and to determine a setting of a regulatory valve in the bypass conduit that extends around the ejector. The control system determines the desired amount of turbine cooling air based on a ratio of the pressure of the cooling air supplied to the second stage of the turbine and the compressor discharge air. This ratio is preferably maintained at a constant set point. To maintain this ratio at the set point, the control system may turn on or off a valve that provides ninth stage compressor air to the ejector and may adjust the valve that regulates the amount of bypass air.
As the control system turns on and off the valve allowing ninth stage compressor air to enter the ejector, there is an immediate change in the pressure ratio of the turbine cooling air to the compressor discharge pressure. The change is due to the addition of ninth stage compressor air to the ejector and hence the turbine cooling air or the termination of such ninth stage compressor air to the ejector. The immediate change in the pressure ratio causes the controller to attempt to adjust the valves to maintain the desired pressure ratio. However, current control systems may not provide controls that adequately address the immediate change in cooling air to compressor discharge pressure ratio. There is a desire, therefore, for a control system that can accommodate rapid variations in a desired turbine cooling air and compressor discharge pressure ratio.